The invention relates to laser systems and laser system components. More particularly, the invention relates to apparatuses and methods for stabilizing a mode-locked laser""s output.
Continuous trains of ultra short laser pulses, e.g., picosecond and femtosecond pulses, are useful in wide range of applications, including materials processing and telecommunications. In most laser systems, such trains of ultra short pulses are generated by either xe2x80x9cpassivelyxe2x80x9d or xe2x80x9cactivelyxe2x80x9d mode-locking a laser.
In passive mode-locking, an element in the laser cavity imposes a nonlinear loss on incident radiation, shaping the energy into a train of pulses. In one popular passive mode-locking scheme, semiconductor-based saturable absorber mirrors are incorporated into a laser cavity. These saturable absorber mirrors include semiconductor layers that, for a particular wavelength range, absorb low intensity radiation, but are relatively transparent to high intensity radiation, such as pulses. Therefore, when incident radiation within the wavelength range encounters the saturable absorber mirror, high intensity pulses penetrate the semiconductor absorptive layers and are reflected by a backmirror, while low intensity radiation is partially absorbed. As a result, a laser that includes a saturable absorber mirror favors pulses over low intensity radiation, and therefore produces a repetitive train of generally equal intensity pulses (a xe2x80x9ccontinuous-wave mode-lockedxe2x80x9d signal), rather than continuous radiation.
In active mode-locking, pulses are generated by imposing a pulse pattern on the laser using an external function generator to drive a modulator. The modulator can be, for example, an electro-optic modulator or an electro-absorption modulator. Typically, the modulation frequency is selected to be a harmonic of the cavity round trip time, in order to maximize the stability of the pulse train.
In both passive and active mode-locking, however, pulse trains can become destabilized by noise or other fluctuations in the laser intensity. In passive mode-locking, perturbations to the system from noise or other sources can cause oscillations in the laser power, or xe2x80x9crelaxation oscillations.xe2x80x9d Unless damped, these relaxation oscillations grow until they transform the continuous-wave mode-locked state into a Q-switched mode-locked state, in which the energy of the pulses varies under a Q-switched envelope. See Kxc3xa4rtner et al., xe2x80x9cControl of Solid State Laser Dynamics by Semiconductor Devices,xe2x80x9d Optical Engineering 34(7): 2024-2036 (1995). In most existing systems, Q-switched mode4ocking is avoided only by increasing the power of the laser.
Similarly, in active mode-locking, if all pulse slots generated by the modulator are not fully filled, other harmonics of the cavity round trip time, known as xe2x80x9csupermodes,xe2x80x9d are excited. If supermodes are excited, the stability of the pulse train is reduced, and the laser""s usefulness is greatly diminished. To suppress supermodes, various techniques, including additive-pulse limiting and self-phase modulation plus filtering, have been attempted. Both additive-pulse limiting and self-phase modulation plus filtering, however, place constraints on the design of the laser system that are undesirable in many applications.
In one aspect, the invention features a laser system that produces radiation at an operative wavelength. The system includes a mode-locking element configured to mode-lock the output of the laser system, and a semiconductor element that produces nonlinear increasing loss at the operative wavelength to enhance stability of the mode-locked output.
Embodiments of this aspect of the invention may include one or more of the following features. The semiconductor element includes a semiconductor material that has a band-edge greater than the laser""s operative wavelength so that, at the operative wavelength, the material exhibits two-photon absorption, but not one-photon absorption. Alternatively, the semiconductor element can include a semiconductor material that has a conduction band, and the material, when exposed to radiation having the operative wavelength, generates sufficient carriers in the conduction band to initiate sufficient free carrier absorption from the conduction band to produce the nonlinear increasing loss.
The system further includes a reflective structure disposed along an optical path in the cavity, and the semiconductor element includes one or more layers of the material disposed on the reflective structure. Alternatively, the semiconductor element can be included on a transmissive structure in the laser cavity.
The laser system can be tunable to produce radiation over a wavelength range, the wavelength range including the operative wavelength. The mode-locking element can be, e.g., a saturable absorber that passively mode-locks the laser system, or an external function generator that actively mode-locks the laser system.
In another aspect, the invention features a laser system that includes a pump, a gain medium, and a reflector disposed along an optical path in the laser""s cavity. The gain medium produces radiation at an operative wavelength when pumped by the pump. The reflector includes one or more layers of a first semiconductor material that acts as a saturable absorber at the operative wavelength to mode-lock the output of the laser, and one or more layers of a second semiconductor material that produces nonlinear increasing loss at the operative wavelength to stabilize the mode-locked output.
Embodiments of this aspect of the invention may include one or more of the following features. The second semiconductor material produces two-photon absorption to achieve the nonlinear increasing loss.
The reflector is configured such that, when light having the operative wavelength is incident upon the reflector, a resulting electric field within the reflector forms a standing wave within the reflector. The standing wave can have a local maximum, e.g., at a location of one or more layers of the first semiconductor material, or at one or more layers of the second semiconductor material.
The second semiconductor material can be, e.g., InP, the first semiconductor material can be, e.g., InGaAs, and the gain medium can be, e.g., an Er/Yb waveguide. The reflector has a dielectric back/mirror and a resonant coating or anti-reflective coating.
In another aspect, the invention features a laser system that includes a pump, a gain medium that produces radiation at an operative wavelength when pumped by the pump, an element that actively mode-locks output of the laser system, and a structure disposed along an optical path in the cavity. The structure includes a semiconductor material that produces nonlinear increasing loss at the operative wavelength to enhance the stability of the mode-locked output.
Embodiments of this aspect of the invention may include one or more of the following features. The material, e.g., InP, produces two-photon absorption to achieve the nonlinear increasing loss. The structure can be, e.g., a reflector that has one or more layers of the material, or a transmissive substrate, such as a waveguide, that includes the material. The gain medium is, e.g., erbium doped fiber.
In another aspect, the invention features a method of enhancing the stability of a continuous wave mode-locked output of a laser that produces radiation at an operative wavelength. The method includes: (a) passively mode-locking the output of the laser to produce a continuous train of pulses; and (b) stabilizing the continuous train of pulses against intensity fluctuations by incorporating into the cavity a semiconductor element that produces a nonlinear increasing loss at the operative wavelength.
Embodiments of this aspect of the invention may include one or more of the following features. The stabilizing step includes stabilizing the continuous train of pulses against Q-switched mode-locking, and the mode-locking step includes mode-locking by incorporating a saturable absorber into the cavity.
In another aspect, the invention features a method of suppressing supermodes in the output of an actively mode-locked laser that produces radiation at an operative wavelength. The method includes: (a) actively mode-locking the laser to produce a continuous train of pulses; and (b) incorporating a semiconductor element into the cavity, the semiconductor element producing a nonlinear increasing loss at the operative wavelength to limit peak intensity of the pulses, and thereby suppress supermodes.
As used herein, the term xe2x80x9cnonlinear increasing loss,xe2x80x9d or xe2x80x9cNIL,xe2x80x9d means a loss that increases as either the peak intensity or pulse energy of the incident radiation increases. An NIL element disposed in a laser cavity, therefore, will produce a loss that increases as the peak intensity or energy of incident pulses increases.
The term xe2x80x9coperative wavelengthxe2x80x9d refers to the wavelength of light produced by a laser system. A laser system that produces light at an operative wavelength can be tunable or non-tunable. If tunable, then the laser system is capable of producing a range of wavelengths that includes the xe2x80x9coperative wavelength.xe2x80x9d
Excited xe2x80x9csupermodesxe2x80x9d in an actively mode-locked laser are harmonics of the cavity round-trip time at frequencies other than the repetition rate or integer multiples of the repetition rate. In a radio frequency spectrum of an actively mode-locked laser, excited super-modes appear as peaks at frequencies other than the frequency of the external function generator, or integer multiples of that frequency.
Other embodiments and advantages of the invention will be apparent from the following description and from the claims.